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Warburton was in a supermarket car park when he heard from Head Coach Warren Gatland last Thursday that he would be taking on the role again, but not before he had hung up on the Kiwi.
The victim of a number of practical jokes played by his Cardiff Blues teammates over the last few weeks, Warburton was determined not to fall for another, putting the phone down on Gatland instead.
He explained: "I found out last Thursday in a supermarket car park when I got the call.
"My wife was getting some bread and milk. The cashier loves rugby so I didn't want to get asked any questions about the Lions.
"I hung up on Gats the first time, I thought it was one of the guys at the Blues messing around with my phone. I've had it every day for the last three weeks, the boys asking me but I genuinely didn't know until Thursday."
Click here for the official 41-man Lions squad
It was Wales winger Tom James who he thought was behind the joke but, thankfully for Warburton, Gatland saw the funny side of it.
He added: "I was in the gym and my phone was unlocked, so when I was driving home I thought, 'Surely the boys have been in my contacts, which they always do, and changed their names to Gats'."
It was Tom James to be exact, the Welsh winger.
"I thought, 'I know TJ has changed his name to Gats', so when it rang and I didn't hear anything I hung up because I thought, 'It's him, messing around up in Merthyr', where he's from. Then it rang again and I heard Gat's Kiwi accent and said, 'Sorry mate, I thought it was TJ messing around'."
In the glamorous setting of the Syon Park Hilton, Tour Manager John Spencer revealed a squad of 41 players who will travel to New Zealand this summer for a ten-match Tour culminating in a three-Test series against the world champions.
There are 16 Englishmen included in the squad with 12 Welshmen, 11 Irishmen and two Scots, while 16 of those players were part of the squad four years ago in Australia.
In order to become the first Lions team to win a series in New Zealand in 46 years, they will have to be victorious in at least one Test at Eden Park - where New Zealand last lost in 1994.
However Gatland believes there are a few areas where the Lions can target the All Blacks, notably in the goal-kicking stakes, as well as up front.
"Looking at the squad, the point of difference is maybe our goal-kicking, I think we've got four or five of the world's best goal-kickers going with us and if we're going to have an edge somewhere it could be in that area," Gatland said.
"You're trying to find areas where you're going to have an advantage and New Zealand always feel that their difference between the rest of the world is their tight five and their ability of the tight five in terms of ball-handling.
"We feel we've got a tight five that can match them, we feel we've got a tight five who are going to be excellent at the set-piece in terms of scrums and lineouts, but also have got the ability to get around the park and have got some good hands and strong ball carrying ability as well.
"The other thing is the ability to match them off the bench, and we feel we've got the players who can come off the bench and have a massive impact too."